E. Koretzky

Characterization of an atmospheric pressure plasma generated by a plasma torch array

Based on a capacitively coupled electrical discharge scheme, it is demonstrated that an array of plasma torches can be lit up simultaneously to form a dense plasma layer in the open air by a single ac power source. The number of torches is only limited by the power handling capability of the source. The measured v–i characteristic of the discharge indicates that the torch is operating in the diffuse arc mode. Experiments have been performed to explore the effect of the plasma torches on the propagation of microwaves in an X-band rectangular waveguide by passing the plasma of the torches through holes in the top and bottom walls of the waveguide. The results show that the plasma torches can effectively attenuate microwaves. The wave–plasma interaction process is also analyzed numerically. The plasma parameters deduced from the theoretical model, by matching the numerical results with those of experiments, are shown to agree well with the experimental measurements.

Design and electrical characteristics of a modular plasma torch

A modular system for constructing an atmospheric pressure plasma source is presented and studied. The design and construction of the plasma torch module by modifying and reassembling the structural components of two different models of spark plugs are first described. Each module can produce a torch plasma of 1 cm radius and 6 cm height and having a peak density exceeding 10/sup 13/ cm/sup -3/. A set of modules, each connected in series with a ballasting capacitor in the circuit, can be operated as an array sharing a common power source to produce a dense and large-volume plasma. The electrical characteristics of the module are studied for the case of a single module and two capacitively coupled modules. The discharge can be maintained, with the aid of ballasting capacitors, in a stable diffuse arc. Furthermore, the coupled discharges in an array of modules are operated with the maximum power efficiency as indicated by a near one power factor seen by the power line.

Temperature measurement of an atmospheric-pressure plasma torch

A simple method to make a temperature measurement in an atmospheric-pressure plasma torch with a density of 1013 electrons/cm3 is developed. The method is based on thermal equilibrium and a detailed analysis of heat loss from a copper wire placed in a torch. The wire diameter, which regulates heat loss, is systematically reduced to increase the temperature of the wire segment in the torch. This process yields a critical wire diameter: Smaller diameters melt and larger diameters are not affected. For the critical diameter the wire temperature in the torch is approximated as the wire melting temperature. This temperature and an analysis of heat input from the torch and heat loss from the wire combine to provide a temperature measurement in the 1300–2200 K range. Using this technique, the temperature of a plasma torch was determined to be approximately 1760 K.

Parametric excitation of lower hybrid waves by Z-mode waves near electron cyclotron harmonics at Tromso

Z-mode wave converted from O-mode heater wave is considered as the pump wave in Tromso HF heating experiments for the parametric excitation of lower hybrid decay mode together with electron Bernstein sidebands in the region above the O-mode reflection layer. This is the process, suggested by Mishin et al. [1997], which generates superthermal electrons by the excited lower hybrid waves. These superthermal electrons produce plasma waves in the region above the O-mode reflection layer, with frequencies much greater than the heater wave frequency, as observed by Isham et al. [1990, 1996]. Our detailed analyses of this process are carried out for two experimental cases wherein the pump wave frequencies are near the third and fifth harmonic electron cyclotron frequencies, respectively. The results show that the minimum threshold field of the instability in either case is less than 0.2 V m−1. However, because the wavelength of the instability is also short, the collisionless dampings restrict the instability occurrence to narrow wavelength and frequency ranges. For 1 V m−1 pump field intensity, the instability can be excited in less than 1 ms. Since the excited lower hybrid waves are strongly Landau damped, superthermal electrons are efficiently generated by them.

A new mechanism of whistler wave generation by amplitude modulated HF waves in the polar electrojet

It is shown that a spatially distributed mode current of whistler waves, oscillating at the modulation frequency of the ground-transmitted powerful HF wave, can be induced in the E-region of the high-latitude ionosphere through the coupling between HF wave-modulated electrojet current and induced density irregularities. As shown in Kuo et. al. [1999], these density irregularities generated by the HF wave via a thermal instability vary periodically along the geomagnetic field. This current produces whistler waves directly rather than through an antenna radiation process. This mechanism generates whistler waves in the VLF range (3–30 KHz) with reduced harmonic components. The frequencies of produced whistler waves depend on the polarization, frequency, power, and modulation scheme of the HF wave. It is predicted that whistler waves at frequencies around 25 KHz can be most favorably excited, awaiting the corroboration of future ionospheric heating experiments.

Generation of density irregularities and whistler waves by powerful radio waves in the polar ionosphere

It is shown that a powerful high-frequency (HF) wave can excite a thermal instability in the E region of the polar ionosphere to introduce a significant electron temperature perturbation. This instability has a broad spatial spectrum with an upper bound. The temperature perturbation then develops nonlinearly, via the electron thermal diffusion process, into spatially periodic irregularities along the geomagnetic field, where the spatial period of the irregularities in the range between 460 m and 1.3 km is mainly governed by the minimum wavelength in the spectrum of the instability. Due to the decreasing dependence on the electron temperature of the recombination rates of electrons with the E region dominant ion species NO+ and O2+, the background plasma density as well as the electrojet current are also perturbed spatially in a similar fashion as the electron temperature irregularities. If an amplitude-modulated HF wave with a modulation frequency in the frequency range between 2 and 30 kHz is used to mod...

Plasma density enhancement and generation of spatially periodic irregularities in the ionospheric E-region by powerful HF waves

A new mechamism is investigated to generate spatially periodic temperature and density irregularities along the geomagnetic field in the ionospheric E-region, using ground transmitted powerful HF (high frequency) waves. A thermal instability is first excited and then stablized by the nonlinear damping from the inelastic electron collisions with N2 and O2. Electron temperature enhanced by powerful HF waves reduces the recombination rates of electrons and NO+ and O2+ ions, and subsequently increases the plasma density. The thermal diffusion of electrons evolves the perturbation into spatially periodic density irregularities along the geomagnetic field. The spatial period between 400 m and 1.6 km of the irregularities varies with the HF wave power and frequency. The optimal conditions for exciting these irregularities are achievable at sites for ionospheric heating experiments. The investigated mechanism works effectively only in the presence of ionization sources, such as UV radiations during the daytime and precipitated energetic particles at night.

Spectral breaking of high-power microwave pulses propagating in a self-induced plasma

An experiment is conducted to confirm the theoretical prediction that a rapidly generated lossy plasma can cause spectral breaking and frequency shift of a high-power microwave pulse. Spectral breaking is the transformation or breaking of a single dominant spectral peak associated with an incident pulse into two spectral peaks. The experiment is conducted by comparing the frequency spectrum of an incident pulse with the spectrum of the pulse transmitted through a self-induced air-breakdown environment. It is shown that as the ionization rate becomes too high, the spectrum of the transmitted pulse breaks up into two peaks: one has an upshifted centre frequency, and the other has a downshifted centre frequency. The results show that the amount of frequency upshift is correlated with the ionization rate, whereas the amount of frequency downshift is correlated with the energy losses from the pulse in the self-generated plasma. These experimental results agree with the theoretical prediction and a numerical simulation, which are also presented.

Simulation study of a capacitively coupled plasma torch array

Summary form only given. It has been shown that an array of air plasma torches can be lit up simultaneously by a single common AC source. (e.g. 60 Hz wall plug) It is achieved by using capacitors as active ballasting circuit elements to prevent voltage shortage in the other electrode pairs due to the discharge in one pair. Charging and discharging of the capacitors provide feedback control to the voltage across the corresponding electrode pair. The setup is modelled as coupled RLC circuits, each electrode pair in the presence of the plasma torch is represented by a parallel RC circuit, in which both R and C are time dependent. The dynamics of the plasma is described by the rate equations of the electrons, positive ions, and negative ions. The coefficients in the rate equations include the ionization frequency, attachment rate, recombination rate, and detachment rate. The ionization frequency, which depends on the discharge field, provides the coupling of the circuit equations to the rate equations of the plasma species.

Design of a modular plasma torch and its interactions with microwaves

The design and construction of the plasma torch module by modifying and reassembling the structural components of two different models of spark plugs are first described. Each module can produce a torch plasma’ of 1 cm radius and 6 cm height and having a peak density exceeding 1013 cmp3. A set of modules, each connected in series with a ballasting capacitor in the circuit, can be operated as an array sharing a common power source to produce a dense and large volume plasma. The electrical characteristics of the module ‘are studied. It’s shown that the discharge can be maintained, with the aid of a series ballasting capacitor, in a stable diffuse arc. Microwave-plasma interaction is studied in an X-band waveguide by passing the plasma from the torch through pairs of aligned holes on the top and bottom walls of the waveguide. The results show that each torch as a lossy dielectric post, can introduce more than 10 dB attenuation to the propagating microwaves.